1. Field of the Invention
This invention relates to an apparatus and process for removing filter cake from filters primarily used for industrial applications, such as removing fly ash from coal-fired flue gas.
2. Description of the Related Art
In the art of filtration, there is a type of filters commonly referred to as “baghouses.” A baghouse is an air pollution abatement device used to trap particulates by filtering gas streams under suction through large fabric tubes called bags. These bags are typically made of glass fibers or fabric. One type of baghouse uses bags that are made of fabrics, such as woven fiberglass, and that are typically 6″, 8″ or 12″ in nominal diameter, with length-to-diameter ratios up to about 60 inches. In operation, the trapped particulates (i.e., “filter cake”) is collected on the inside of the bags and subsequently removed. Removal typically is effected by a reverse flow process (“backwash”) of clean gas.
In one version of baghouse design, the bottom of the bag is open, the top of the bag is closed, and the dirty gas flow is up into the bag. This is known as the “bottom inlet reverse flow baghouse” or (BI).
Another type of baghouse design is the same as the (BI), except that both ends of the bag are open, the dirty gas enters the top of the bags, and the dirty gas flow is down in the bag. This design is known as the “top inlet reverse flow baghouse” or (TI).
The third type of baghouse uses bags that collect filtrate on the outside of the bag, which are open at the top, closed at the bottom, and cleaned by reverse flow of bursts of compressed air. This design is known as a “pulse—jet baghouse” or (PJ).
Bag collapse during reverse flow cleaning is limited in BIs and TIs by anti-collapse metal rings sewn at variable intervals on the full length of the outside of the bags. PJ bags are supported by internal wire cages that limit collapse when the bags are on line.
BIs are usually cleaned one compartment at a time and return the dirty reverse “air” to the inlet duct ahead of the downstream compartments. The compartments are usually arranged in two parallel rows, both rows served by a common inlet duct, which also receives the dirty gas leaving the compartment being cleaned. The first two compartments in line never see any dirty reverse flow, which is cleaned by the other compartments.
With one or more of the BI compartments off line for cleaning there is variable flow through the baghouse that has to be accommodated by the plant control system. The flow rate for a newly cleaned BI compartment is high and decreases with time on line. The flow rate entering the bottom of a BI bag decreases to zero at the top of the bag because the gas volume rising in the bag decreases as gas leaves the bag. Therefore, in multi-compartment BIs, each compartment operates at a different and variable flow rate.
PJ compartments operate at constant flow rate that does not vary with time on line, and PJ bags are cleaned on line. The flow rate entering the bags is high at the bottom of the bags and decreases to zero at the top of the bags. However, some of the cake blown off during cleaning is recaptured and refiltered by adjacent on line bags.
Similarly, in BIs, some of the particulate (filter cake) is refiltered a variable number of times and in variable amounts depending on the position of the compartment in the baghouse.
The filter cake in both BIs and PJs varies in cake thickness and particle size distribution over the length of every bag. The filter cake in TIs has uniform particle size distribution over the full length of the every bag. TI cake permeability is thought to be uniform over the full length of every bag and decreases with time on line.
The present state-of-the-art for TIs requires off-line cleaning which varies the amount of cloth in service, affects the plant control system, and necessitates some refiltering.
While suitable for their intended purpose, BIs and PJs have some disadvantages as described above. TIs have none of these faults and do have some features that are not possible with the other baghouse designs. Thus, there continues to be a need for improved bag cleaning, especially utilizing a TI design.